In several forms of β-thalassemia, mutations in the second intron of the β-globin gene create aberrant 5′ splice sites and activate a common cryptic 3′ splice site upstream. As a result, the thalassemic β-globin pre-mRNAs are spliced almost exclusively via the aberrant splice sites leading to a deficiency of correctly spliced β-globin mRNA and, consequently, β-globin. We have designed a series of vectors that express modified U7 snRNAs containing sequences antisense to either the aberrant 5′ or 3′ splice sites in the IVS2–705 thalassemic pre-mRNA. Transient expression of modified U7 snRNAs in a HeLa cell line stably expressing the IVS2–705 β-globin gene restored up to 65% of correct splicing in a sequence-specific and dose-dependent manner. Cell lines that stably coexpressed IVS2–705 pre-mRNA and appropriately modified U7 snRNA exhibited up to 55% of permanent restoration of correct splicing and expression of full-length β-globin protein. This novel approach provides a potential alternative to gene replacement therapies.
Ornithine transcarbamylase (ornithine carbamoyltransferase; carbamoyl-phosphate:L-ornithine carbamoyltransferase, EC 188.8.131.52) is a mitochondrial matrix enzyme of the mammalian urea cycle. The X chromosome-linked spfash mutation in the mouse causes partial ornithine transcarbamylase deficiency and has served as a model for the human disease. We show here that the spfash mutation is a guanine to adenine transition of the last nucleotide of the fourth exon of the ornithine transcarbamylase gene. This nucleotide change produces two remarkably different effects. First, this transition causes ornithine transcarbamylase mRNA deficiency because the involved exon nucleotide plays a part in the recognition of the adjacent splice donor site. As a result of the mutation, ornithine transcarbamylase pre-mRNA is spliced inefficiently both at this site and at a cryptic splice donor site 48 bases into the adjacent intron. Second, two mutant proteins are translated from these mRNAs. From the correctly spliced mRNA, the transition results in a change of amino acid 129 from arginine to histidine. This missense substitution has no discernable effect on mitochondrial import, subunit assembly, or enzyme activity. On the other hand, the elongated mRNA resulting from mis-splicing is translated into a dysfunctional ornithine transcarbamylase subunit elongated by the insertion of 16 amino acid residues.
Platelet-derived growth factor (PDGF) is a cationic dimer composed of two chains, designated A and B. All three dimeric isotypes of PDGF, PDGF-AA, -AB and -BB, are biologically active but may have distinct functional activities. Two A-chain precursors which differ by the presence of a highly basic 15 amino acid C-terminal extension are derived from the A-chain by alternative RNA splicing. To compare the functional properties of these two different forms of the A-chain, expression vectors were generated in which the cDNAs were placed under the transcriptional control of a viral promoter (pSV2). Surprisingly, cryptic RNA splice donor sites were identified in both forms of the PDGF A-chain which modify the A-chain open reading frame and alter the structure of the expressed protein. Recognition of this phenomenon appears to explain the discrepancies between previous results regarding the secretory properties of the PDGF A-chain and may explain difficulties in expression vectors containing splice acceptor sites between the inserted sequence and the polyadenylation site.
(SLS) is an autosomal recessive disorder characterized by ichthyosis,
mental retardation, spasticity, and deficient activity of fatty aldehyde
dehydrogenase (FALDH). To define the molecular defects causing SLS, we
performed mutation analysis of the FALDH gene in probands from 63 kindreds
with SLS. Among these patients, 49 different mutations—including 10
deletions, 2 insertions, 22 amino acid substitutions, 3 nonsense mutations,
9 splice-site defects, and 3 complex mutations—were found. All
of the patients with SLS were found to carry mutations. Nineteen of the
missense mutations resulted in a severe reduction of FALDH enzyme catalytic
activity when expressed in mammalian cells, but one mutation (798G→C
[K266N]) seemed to have a greater effect on mRNA stability. The
splice-site mutations led to exon skipping or utilization of cryptic
acceptor-splice sites. Thirty-seven mutations were private,
and 12 mutations were seen in two or more probands of European or Middle
Eastern descent. Four single-nucleotide polymorphisms (SNPs) were
found in the FALDH gene. At least four of the common mutations
(551C→T, 682C→T, 733G→A, and 798+1delG) were associated with
multiple SNP haplotypes, suggesting that these mutations originated
independently on more than one occasion or were ancient SLS genes that
had undergone intragenic recombination. Our results demonstrate that SLS
is caused by a strikingly heterogeneous group of mutations in the FALDH
gene and provide a framework for understanding the genetic basis of SLS and
the development of DNA-based diagnostic
The process of excising introns from pre-mRNA complexes is directed by specific genomic DNA sequences at intron—exon borders known as splice sites. These regions contain well-conserved motifs which allow the splicing process to proceed in a regulated and structured manner. However, as well as conventional splicing, several genes have the inherent capacity to undergo alternative splicing, thus allowing synthesis of multiple gene transcripts, perhaps with different functional properties. Within the human genome, therefore, through alternative splicing, it is possible to generate over 100,000 physiological gene products from the 35,000 or so known genes. Abnormalities in normal or alternative splicing, however, account for about 15% of all inherited single gene disorders, including many with a skin phenotype. These splicing abnormalities may arise through inherited mutations in constitutive splice sites or other critical intronic or exonic regions. This review article examines the process of normal intron—exon splicing, as well as what is known about alternative splicing of human genes. The review then addresses pathological disruption of normal intron—exon splicing that leads to inherited skin diseases, either resulting from mutations in sequences that have a direct influence on splicing or that generate cryptic splice sites. Examples of aberrant splicing...
Sequences governing RNA splicing are difficult to study in situ due to the great difficulty of traditional targeted mutagenesis. Zinc-finger nuclease (ZFN) technology allows for the rapid and efficient introduction of site-specific mutations into mammalian chromosomes. Using a ZFN pair along with a donor plasmid to manipulate the outcomes of DNA repair, we introduced several discrete, targeted mutations into the fourth intron of the endogenous BAX gene in Chinese hamster ovary cells. Putative lariat branch points, the polypyrimidine tract, and the splice acceptor site were targeted. We recovered numerous otherwise isogenic clones carrying the intended mutations and analyzed the effect of each on BAX pre-mRNA splicing. Mutation of one of three possible branch points, the polypyrimidine tract, and the splice acceptor site all caused exclusion of exon five from BAX mRNA. Interestingly, these exon-skipping mutations allowed usage of cryptic splice acceptor sites within intron four. These data demonstrate that ZFN-mediated gene editing is a highly effective tool for dissection of pre-mRNA splicing regulatory sequences in their endogenous context.
A large fraction of sequence variants of unknown significance (VUS) of the breast and ovarian cancer susceptibility genes BRCA1 and BRCA2 may induce splicing defects. We analyzed 53 VUSs of BRCA1 or BRCA2, detected in consecutive molecular screenings, by using five splicing prediction programs, and we classified them into two groups according to the strength of the predictions. In parallel, we tested them by using functional splicing assays. A total of 10 VUSs were predicted by two or more programs to induce a significant reduction of splice site strength or activation of cryptic splice sites or generation of new splice sites. Minigene-based splicing assays confirmed four of these predictions. Five additional VUSs, all at internal exon positions, were not predicted to induce alterations of splice sites, but revealed variable levels of exon skipping, most likely induced by the modification of exonic splicing regulatory elements. We provide new data in favor of the pathogenic nature of the variants BRCA1 c.212+3A>G and BRCA1 c.5194−12G>A, which induced aberrant out-of-frame mRNA forms. Moreover, the novel variant BRCA2 c.7977−7C>G induced in frame inclusion of 6 nt from the 3′ end of intron 17. The novel variants BRCA2 c.520C>T and BRCA2 c.7992T>A induced incomplete skipping of exons 7 and 18...
DMD nonsense and frameshift mutations lead to severe Duchenne muscular dystrophy while in-frame mutations lead to milder Becker muscular dystrophy. Exceptions are found in 10% of cases and the production of alternatively spliced transcripts is considered a key modifier of disease severity. Several exonic mutations have been shown to induce exon-skipping, while splice site mutations result in exon-skipping or activation of cryptic splice sites. However, factors determining the splicing pathway are still unclear. Point mutations provide valuable information regarding the regulation of pre-mRNA splicing and elements defining exon identity in the DMD gene. Here we provide a comprehensive analysis of 98 point mutations related to clinical phenotype and their effect on muscle mRNA and dystrophin expression. Aberrant splicing was found in 27 mutations due to alteration of splice sites or splicing regulatory elements. Bioinformatics analysis was performed to test the ability of the available algorithms to predict consequences on mRNA and to investigate the major factors that determine the splicing pathway in mutations affecting splicing signals. Our findings suggest that the splicing pathway is highly dependent on the interplay between splice site strength and density of regulatory elements.
Mutation-induced activation of splice sites in intronic repetitive sequences has contributed significantly to the evolution of exon–intron structure and genetic disease. Such events have been associated with mutations within transposable elements, most frequently in mutation hot-spots of Alus. Here, we report a case of Alu exonization resulting from a 367-nt genomic COL4A5 deletion that did not encompass any recognizable transposed element, leading to the Alport syndrome. The deletion brought to proximity the 5′ splice site of COL4A5 exon 33 and a cryptic 3′ splice site in an antisense AluY copy in intron 32. The fusion exon was depleted of purines and purine-rich splicing enhancers, but had low levels of intramolecular secondary structure, was flanked by short introns and had strong 5′ and Alu-derived 3′ splice sites, apparently compensating poor composition and context of the new exon. This case demonstrates that Alu splice sites can be activated by outlying deletions, highlighting Alu versatility in shaping the exon–intron organization and expanding the spectrum of mutational mechanisms that introduce repetitive sequences in mRNAs.
Group II introns are catalytic RNAs that are excised from their precursors in a protein-dependent manner in vivo. Certain group II introns can also react in a protein-independent manner under nonphysiological conditions in vitro. The efficiency and fidelity of the splicing reaction is crucial, to guarantee the correct formation and expression of the protein-coding mRNA. RmInt1 is an efficient mobile intron found within the ISRm2011-2 insertion sequence in the symbiotic bacterium Sinorhizobium meliloti. The RmInt1 intron self-splices in vitro, but this reaction generates side products due to a predicted cryptic IBS1* sequence within the 3′ exon. We engineered an RmInt1 intron lacking the cryptic IBS1* sequence, which improved the fidelity of the splicing reaction. However, atypical circular forms of similar electrophoretic mobility to the lariat intron were nevertheless observed. We analyzed a run of four cytidine residues at the 3′ splice site potentially responsible for a lack of fidelity at this site leading to the formation of circular intron forms. We showed that mutations of residues base-pairing in the tertiary EBS3–IBS3 interaction increased the efficiency and fidelity of the splicing reaction. Our results indicate that RmInt1 has developed strategies for decreasing its splicing efficiency and fidelity. RmInt1 makes use of unproductive splicing reactions to limit the transposition of the insertion sequence into which it inserts itself in its natural context...
Lynch syndrome, a heritable form of cancer predisposition, is caused by germline mutations within genes of the DNA mismatch repair family, and can be rapidly identified in young onset cancer patients through the detection of loss of expression of at least one of these genes in tumour samples. To date, such causative mutations have only been identified within exonic and splice site regions. Though this approach has been successful in the majority of families, a considerable number remain in which no mutation has been found. To address this situation, we used an alternative mutation discovery procedure which involved haplotype analysis of the locus containing the gene lost in the tumour and delineation of segregating haplotypes, followed by an investigation of splicing aberrations to uncover cryptic splice sites which lay outside the genomic regions routinely examined for mutations. In this report, we show that an intronic mutation 478 bp upstream of exon 2 in the MSH2 gene causes Lynch syndrome through creation of a novel splice donor site with subsequent pseudoexon activation, thus highlighting the need for more extensive sequencing approaches in families where routine procedures fail to find a mutation.
Juan-Mateu, Jonàs; González-Quereda, Lidia; Rodríguez, María José; Verdura, Edgard; Lázaro, Kira; Jou, Cristina; Nascimento, Andrés; Jiménez-Mallebrera, Cecilia; Colomer, Jaume; Monges, Soledad; Lubieniecki, Fabiana; Foncuberta, María Eugenia; Pa
Fonte: Public Library of SciencePublicador: Public Library of Science
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
DMD nonsense and frameshift mutations lead to severe Duchenne muscular dystrophy while in-frame mutations lead to milder Becker muscular dystrophy. Exceptions are found in 10% of cases and the production of alternatively spliced transcripts is considered a key modifier of disease severity. Several exonic mutations have been shown to induce exon-skipping, while splice site mutations result in exon-skipping or activation of cryptic splice sites. However, factors determining the splicing pathway are still unclear. Point mutations provide valuable information regarding the regulation of pre-mRNA splicing and elements defining exon identity in the DMD gene. Here we provide a comprehensive analysis of 98 point mutations related to clinical phenotype and their effect on muscle mRNA and dystrophin expression. Aberrant splicing was found in 27 mutations due to alteration of splice sites or splicing regulatory elements. Bioinformatics analysis was performed to test the ability of the available algorithms to predict consequences on mRNA and to investigate the major factors that determine the splicing pathway in mutations affecting splicing signals. Our findings suggest that the splicing pathway is highly dependent on the interplay between splice site strength and density of regulatory elements
We present evidence that mammalian cells have the ability to generate functional mRNA molecules by trans-splicing. Rat cells, transformed by an early SV40 DNA fragment (Bst/Bam) synthesize a truncated T antigen (T1 antigen), although the cells do not have a direct sequence homology for the T1 antigen at the DNA level. The Bst/Bam DNA fragment encodes exclusively for the second SV40 T antigen exon (aa 83-708) and contains the entire small t antigen intron. To synthesize the corresponding mRNA (T1 mRNA), the cells utilize a cryptic 5' splice site within the second exon (codons for aa 131/132) as donor site and the upstream small t antigen 3' splice site as the acceptor site. Since these sites are in an inverted order on the pre-mRNA, two Bst/Bam transcripts are required to generate one T1 mRNA molecule. HeLa cell nuclear extracts also performed the trans-splicing reaction in vitro.
The defective Suppressor-mutator (dSpm)-induced allele bronze1-mutable 13 (bz1-m13) and many of its derivative alleles are leaky mutants with measurable levels of flavonol O3-glucosyltransferase activity. This activity results from splicing at acceptor site-1, one of two cryptic 3' splice sites within the dSpm insertion in bz1-m13. In this study, splicing in bz1-m13 change-in-state (CS) alleles CS-3 and CS-64 was shown to be altered from bz1-m13; previous work found altered splicing in CS-9. CS-64 is a null allele and lacks the acceptor site-1-spliced transcript because this site is deleted. CS-3 and CS-9 had increased levels of the acceptor site-1 transcript relative to bz1-m13 and increased enzymic activities. A deletion in CS-9 altered splicing by eliminating acceptor site-2. Both acceptor sites were intact in CS-3, but a deletion removed most of a 275-bp GC-rich sequence in dSpm. This suggests that GC-rich sequences affect splicing and is consistent with models postulating a role for AU content in the splicing of plant introns. Splicing does not necessarily occur, however, at the junction of AU-rich intron sequences and GC-rich exon sequences.
Antisense 2'-O-methylribooligonucleotides were targeted against specific sequence elements in mutated human beta-globin pre-mRNAs to restore correct splicing of these RNAs in vitro. The following mutations of the beta-globin gene, A-->G at nt 110 of the first intron (beta 110), T-->G at nt 705 and C-->T at nt 654 of the second intron (IVS2(705) and IVS2(654), respectively), which led to aberrant splicing of the corresponding pre-mRNAs, were previously identified as the underlying causes of beta-thalassemia. Aberrant splicing of beta 110 pre-mRNA was efficiently reversed by an oligonucleotide targeted against the branch point sequence in the first intron of the pre-mRNA but not by an oligonucleotide targeted against the aberrant 3' splice site. In both IVS2(705) and IVS2(654) pre-mRNAs, correct splicing was restored by oligonucleotides targeted against the aberrant 5' splice sites created by the mutations in the second intron or against a cryptic 3' splice site located upstream and activated in the mutated background. These experiments represent an approach in which antisense oligonucleotides are used to restore the function of a defective gene and not, as usual, to down-regulate the expression of an undesirable gene.
We isolated and characterized three spontaneous mutants of Chinese hamster ovary cells that were deficient in dihydrofolate reductase activity. All three mutants contained no detectable enzyme activity and produced dihydrofolate reductase mRNA species that were shorter than those of the wild type by about 120 bases. Six exons are normally represented in this mRNA; exon 5 was missing in all three mutant mRNAs. Nuclease S1 analysis of the three mutants indicated that during the processing of the mutant RNA, exon 4 was spliced to exon 6. The three mutant genes were cloned, and the regions around exons 4 and 5 were sequenced. In one mutant, the GT dinucleotide at the 5' end of intron 5 had changed to CT. In a second mutant, the first base in exon 5 had changed from G to T. In a revertant of this mutant, this base was further mutated to A, a return to a purine. Approximately 25% of the mRNA molecules in the revertant were spliced correctly to produce an enzyme with one presumed amino acid change. In the third mutant, the AG at the 3' end of intron 4 had changed to AA. A mutation that partially reversed the mutant phenotype had changed the dinucleotide at the 5' end of intron 4 from GT to AT. The splicing pattern in this revertant was consistent with the use of cryptic donor and acceptor splice sites close to the original sites to produce an mRNA with three base changes and a protein with two amino acid changes. These mutations argue against a scanning model for the selection of splice site pairs and suggest that only a single splice site need be inactivated to bring about efficient exon skipping (a regulatory mechanism for some genes). The fact that all three mutants analyzed exhibited exon 5 splicing mutations indicates that these splice sites are hot spots for spontaneous mutation.
Ornithine transcarbamylase (OTC) is an X-linked, liver-specific enzyme that catalyzes the second step of the urea cycle. In humans, inherited deficiency of OTC in hemizygous affected males usually results in severe ammonia intoxication and early death. To characterize mutations responsible for OTC deficiency, we used the PCR to amplify cDNAs prepared from patient livers which demonstrated no OTC enzyme activity and no OTC cross-reacting material on western blots. In three of seven cases, smaller than normal products were observed. Sequencing of these cDNAs revealed that two were missing exon 7 of the OTC gene and that the other was missing the first 12 bp of exon 5. Sequencing of genomic DNA from these three patients revealed that one mutant missing exon 7 had a T-to-C substitution in the 5' splice donor site of intron 7. The other mutant missing exon 7 had an A-to-G change in the third position of intron 7. It is interesting that both of these mutations resulted in skipping the preceding exon rather than in inclusion of some or all of the affected intron. In the third mutant, an A-to-T substitution was found in the 3' splice acceptor site at the end of intron 4. Here, a cryptic splice acceptor site within exon 5 was used. Northern blotting of liver RNA from these patients demonstrated (a) reduced...
The mechanism of cellular src (c-src) transduction by a transformation-defective deletion mutant, td109, of Rous sarcoma virus was studied by sequence analysis of the recombinational junctions in three td109-derived recovered sarcoma viruses (rASVs). Our results show that two rASVs have been generated by recombination between td109 and c-src at the region between exons 1 and 2 defined previously. Significant homology between td109 and c-src sequences was present at the sites of recombination. The viral and c-src sequence junction of the third rASV was formed by splicing a cryptic donor site at the 5' region of env of td109 to exon 1 of c-src. Various lengths of c-src internal intron 1 sequences were incorporated into all three rASV genomes, which resulted from activation of potential splice donor and acceptor sites. The incorporated intron 1 sequences were absent in the c-src mRNA, excluding its being the precursor for recombination with td109 and implying that initial recombinations most likely took place at the DNA level. A potential splice acceptor site within the incorporated intron 1 sequences in two rASVs was activated and was used for the src mRNA synthesis in infected cells. The normal env mRNA splice acceptor site was used for src mRNA synthesis for the third rASV.
A genomic DNA probe derived from the region immediately 3' of the clusters of integrated proviruses in the Mlvi-4 locus detects a 5.5-kb mRNA transcript which is specifically expressed in normal rat thymus and spleen. The same probe detects two tumor-specific mRNA transcripts 2.5 and 10 kb long, both of which are expressed only in tumors carrying a provirus in the Mlvi-4 locus. Sequence analysis of two cDNA clones (LE3a and B1.1) of the 2.5-kb tumor-specific mRNA, obtained from two independent tumors (6889 and B1), revealed that they are both derived from hybrid env/Mlvi-4 mRNA transcripts. The splicing of env to Mlvi-4 sequences linked a cryptic splice donor site at nucleotide position 6397 of the viral genome with a splice acceptor site in the region immediately 3' of the integrated provirus. The mRNA that gives rise to cDNA clone B1.1 terminates 1,005 bases 3' of the splice acceptor site without additional splicing. The mRNA that gives rise to cDNA clone LE3a terminates in the same site but undergoes differential splicing of an 81-base-long intron. The resulting mRNAs contain 247-amino-acid (clone B1.1) or 226-amino-acid (clone LE3a) open reading frames sharing 221 N-terminal amino acids, of which 207 are derived from the viral env gene and 14 are derived from Mlvi-4. RNase protection assays using 6889 tumor cell RNA and a probe derived from the cDNA clone LE3a detected both mRNA transcripts. More abundant of the two...
Very small vertebrate exons are problematic for RNA splicing because of the proximity of their 3' and 5' splice sites. In this study, we investigated the recognition of a constitutive 7-nucleotide mini-exon from the troponin I gene that resides quite close to the adjacent upstream exon. The mini-exon failed to be included in spliced RNA when placed in a heterologous gene unless accompanied by the upstream exon. The requirement for the upstream exon disappeared when the mini-exon was internally expanded, suggesting that the splice sites bordering the mini-exon are compatible with those of other constitutive vertebrate exons and that the small size of the exon impaired inclusion. Mutation of the 5' splice site of the natural upstream exon did not result in either exon skipping or activation of a cryptic 5' splice site, the normal vertebrate phenotypes for such mutants. Instead, a spliced RNA accumulated that still contained the upstream intron. In vitro, the mini-exon failed to assemble into spliceosome complexes unless either internally expanded or accompanied by the upstream exon. Thus, impaired usage of the mini-exon in vivo was accompanied by impaired recognition in vitro, and recognition of the mini-exon was facilitated by the presence of the upstream exon in vivo and in vitro. Cumulatively...